Urban bird declines and the fear of cats

The role of domestic cats Felis catus in the troubling, on-going decline of many urban bird populations in the UK is controversial. Debate, in the UK and elsewhere, has centred on the level of avian mortality directly imposed by cats, and on whether this is principally compensatory (the 'doomed surplus' hypothesis) or additive (the 'hapless survivor' hypothesis). However, it is well established that predators also have indirect, sub-lethal effects on their prey where life-history responses to predation risk affect birth and death rates. Here, using a simple model combining cat predation on birds with a sub-lethal (fear) effect of cat density on bird fecundity, we show that these sub-lethal effects may be substantial for urban songbirds. When cat densities are as high as has been recorded in the UK, and even when predation mortality is low (e.g. < 1%), a small reduction in fecundity due to sub-lethal effects (e.g. < 1 offspring year(-1) cat(-1)) can result in marked decreases in bird abundances (up to 95%). Thus, low predation rates in urban areas do not necessarily equate with a correspondingly low impact of cats on birds. Sub-lethal effects may depress bird populations to such an extent that low predation rates simply reflect low prey numbers.